The present invention relates, generally, to liquid crystal display assemblies and, more particularly, relates to miniature liquid crystal display assemblies constructed to reduce residual stresses.
In the recent past, substantial research and development resources have been directed toward small scale Liquid Crystal Display (LCD) and light valve technologies. These high information content, miniature LCD assemblies enable enhanced availability of graphics, data and video information for employment in high resolution projection displays, such as a reflective LCD projectors, SXGA formats (1,280xc3x971,024 pixel resolution) and even HDTV formats (above 1,000 line resolution), or the like.
Reflective LCD projectors, in particular, are highly desirable since they offer the brightness of traditional three-lamp front-projection systems in combination with the high resolution of an LCD panel. At the heart of these optical engines is reflective liquid crystal on crystalline silicon light valve technology which, when combined with sophisticated optical architecture and the appropriate electronic interface, enables very high resolution, high brightness, large screen displays.
One problem associated with both transmissive and reflective-type LCD panels assemblies is bowing or warpage of individual panels caused by residual stresses acting upon the components during operation. This is particularly noticeable in reflective-type LCD panels which have increased flatness requirements due to the nature of the reflective surface of the die. For example, thermal expansion characteristics, as well as lattice mismatching, can generate significant stresses in the underlying substrate material (the silicon), therein causing significant bowing of the mirrored surface. The bowing, which translates to a non-planarity of the surface, causes both (1) a non-uniform thickness of the liquid crystal layer between the bowed reflective surface and the planar transmissive top layer, and (2) variations in the path length of the reflected light from different parts of the element, and of the array. These effects compromise the electro-optic properties of the elements and/or array.
Typically, a primary source of these residual stresses originate from the different materials and composites of the LCD panel having different coefficients of expansion. This is best shown in FIGS. 1 and 2 which illustrate a flex circuit device 18 electrically coupling a conventional small scale LCD assembly 20 to an electronic interface (not shown). The LCD assembly 20 includes a die 21 having a pixel array 22 which is generally composed of rows and columns of electrically conductive pathways each forming an individual pixel (not shown). Each pixel can be individually changed to an xe2x80x9conxe2x80x9d condition by selecting the appropriate row and column of pixel array 22. Positioned around or concentrated on one end of the pixel array are a plurality of die bond pads 23 which are internally connected to the pixel array 22 to enable operational control thereof. Selection of the appropriate pixel is controlled by control circuitry, either included within the die 21 or external to the die 21. In either configuration, external control signals may be used to control the functions of the die 21.
A transparent glass plate 24 is typically placed over the die 21 and the pixel array 22, such that a portion of the glass plate 24 overhangs the die 21. The glass plate 24 is usually affixed to die 21 through an adhesive seal 25 which together cooperate to define a sealed volume encompassing the pixel array 22. This sealed volume is then commonly filled with a solution 26 of Polymer Dispersed Liquid Crystals (PDLC). To facilitate grounding or the application of a charge across the face of the glass plate 24, a conductive coating 37 may be deposited over the undersurface 28 thereof.
The die 21 is typically rigidly or semi-rigidly mounted to a substrate 27 for mounting support and heat conductive dissipation for the die. A conductive adhesive 29 (FIG. 2), such as a conductive epoxy or a thermally conductive non-adhesive grease, is generally applied to the undersurface of the die 21 to affix the die directly to the top surface of the substrate 27. Accordingly, a heat conductive pathway is created directly between the die and the substrate to dissipate heat generated by the die.
The flex circuit 18 includes a plurality of flex circuit bond pads 30 which are typically wire bonded to the die bond pads 23 through bonding wires 31. More recently, a distal ringed coupling portion 32 of flex circuit 18 is adhesively or fixedly mounted to the top surface of substrate 27 for support thereof. Finally, a glob coating 33 is applied to die 21, substrate 27 and the distal end of flex circuit 18. The glob coating 33 (FIG. 3) further normally encapsulates the bonding wires 31 and the die and flex circuit bonding pads 23 and 30 without obscuring a view of the pixel array 22 through the glass plate 24.
As previously indicated, one important aspect in the proper operation of these small scale LCD or light valve assemblies is the maintenance of proper distance uniformity (preferably about 2-4xcexcm) between the pixel array and the undersurface 28 of the glass plate. Variances in the separation of the glass plates may often times cause the pixel array to function improperly or cause operational failure.
Conventional rigid display device constructions, for example, often warp during operation since the substrate 27, the glass plate 24 and the silicon die 21 are all composed of materials or composites having different coefficients of expansion. The individual components of the LCD assembly, therefore, often expand at different degrees and rates. Further, depending in part upon the construction processes, such as the adhesive curing techniques, significant residual stresses may be induced upon the cell. Eventually, in severe instances, the glass plate 24 may delaminate from the die 21. At a minimum, these internal stresses cause optical defects such as variations in color uniformity and fringes, and variations in the cell gap thickness may cause optical shadows.
One particular cause of residual stresses is the formation of an electrical connection between the transparent glass plate 24 and a circuit 36 of the flex circuit 18 which may be employed for applying a charge or the like of the glass plate. A conductive coating 37 of indium tin oxide is typically deposited over the undersurface 28 of the glass plate 24 to facilitate electrical connection therewith. As best viewed in FIG. 3, an indium solder bump or tab 38 is pre-bonded to the indium coating 37 to enhance electrical coupling therewith. A conductive kovar slug 40 is positioned between the solder tab 38 and the circuit 36 which in turn is bonded therebetween using a conductive epoxy 41 or the like.
While this approach sufficiently electrically couples the transparent glass 24 to the flex circuit 18, the electrical connection is substantially rigid in nature. Hence, the epoxies 41, the kovar slug 40 and/or the indium solder tab 38 expand or contract during operation thereof, residual stresses are imparted upon the transparent glass plate by the rigid connection to the circuit 36, which in turn is fixedly mounted to the rigid substrate 27. For example, upon curing of the epoxy 41, shrinkage occurs which imparts a residual stress upon the glass 24.
Accordingly, there is a need to provide a LCD assembly which minimizes residual stress induced upon the cell.
The present invention provides a connection assembly adapted to electrically couple a transparent plate of a liquid crystal display device to an operating subsystem. The connection assembly includes an elongated flexible tape member having an elongated metallic circuit. This metallic circuit includes a main circuit portion fixedly mounted to the tape member, and a lead terminal. This terminal includes a contact portion adapted to electrically couple to the transparent plate, and a substantially flexible and movable conductive joint portion. Collectively, the contact portion and the conductive joint portion electrically couple the contact portion to the circuit portion of the metallic circuit in a manner substantially minimizing residual stresses formed from the electrical coupling between the transparent plate and the circuit portion.
In one embodiment, the conductive joint is provided by a section of the main circuit portion which is physically separated from fixed mounting to the tape member. Hence, the conductive joint is essentially part of the metallic main circuit portion separated from the polyamide tape member which enables substantial movement freedom by the conductive joint for a substantially stress-free electrical connection.
In another configuration, a liquid crystal display assembly is provided including a die having a pixel array and a plurality of bond pads in electrical communication with the pixel array. The display assembly further includes a transparent plate, and an adhesive seal adhesively coupling the die to the transparent plate. The adhesive seal, the transparent plate and the die cooperate to define a sealed volume therebetween encompassing the pixel array upon which a liquid crystal material is disposed within. A flex circuit device includes a flexible tape member, a plurality of primary circuits and a second circuit. The primary circuits terminate at respective first terminals of a coupling region thereof which are electrically coupled to the bond pads of the die. The second circuit includes a main circuit portion fixedly mounted to the tape member and terminates at a second terminal. The second terminal includes a contact portion adapted to electrically couple to the transparent plate and a substantially flexible and movable conductive joint portion electrically coupling the contact portion to the circuit portion of the second circuit in a manner substantially minimizing residual stresses formed from the electrical coupling between the transparent plate and the circuit portion.
In one embodiment of the liquid crystal display assembly, a substrate is provided which is coupled to a backside surface of the die. Moreover, the coupling region of the tape member, which preferably defines a receptacle portion formed and dimensioned for receipt of the liquid crystal display device therein, is fixedly mounted to the substrate.
In this arrangement, the conductive joint may be provided by a section of the main circuit portion which is physically separated from fixed mounting to the tape member and the substrate. This freedom of movement of the conductive joint enable a substantially stress-free electrical connection with the transparent plate.
In yet another embodiment, the substrate includes an aperture formed and dimensioned for receipt of a probe therethrough. During connection of the conductive joint to the transparent plate, a probe may be positioned through the aperture to contact a backside of the conductive joint an urge the same into heated contact against the transparent plate at a bond site for bonding thereof. Preferably, the probe is provided by a heated thermode, and the contact portion of the thermal bond conductive material forms a thermal bond between the transparent plate and the second terminal when contacted by the heated thermode.
In still another embodiment of the present invention, a method of packaging a liquid crystal display assembly is provided including providing a display device including a die having a pixel array, a transparent plate, an adhesive seal adhesively coupling the die to the transparent plate, and a liquid crystal material disposed within a sealed volume formed between the adhesive seal, the transparent plate and the die. The method further includes providing an elongated flexible tape member having a main circuit portion fixedly mounted thereto, and having a lead terminal thereof. The method of the present invention further includes electrically connecting a contact portion of the lead terminal to the transparent plate through a substantially flexible and movable conductive joint portion of the lead terminal to substantially minimize residual stresses formed from the electrical coupling between the transparent plate and the circuit portion.
Another embodiment of the method of the present invention includes forming the conductive joint by physically separating a section of the main circuit portion from fixed mounting to the tape member to enable the substantial flexibility thereof for a substantially stress-free electrical connection. The method further includes thermal bonding, or adhering through an electrically conductive pressure sensitive adhesive, the contact portion to transparent plate.
In another aspect of the present invention, a method of packaging a liquid crystal display assembly is included comprising: providing a display device including a die having a pixel array, a transparent plate, an adhesive seal adhesively coupling the die to the transparent plate, and a liquid crystal material disposed within a sealed volume formed between the adhesive seal, the transparent plate and the die. The method of the present invention further includes providing an elongated flexible tape member including a main circuit portion fixedly mounted thereto, and separating a section of the main circuit portion from fixed mounting to the tape member to form a lead terminal having a contact portion and a substantially flexible and movable conductive joint portion therebetween. The inventive method further includes electrically connecting the contact portion of the lead terminal to the transparent plate such that the substantially flexible and movable conductive joint portion substantially minimizes residual stresses formed from the electrical coupling between the transparent plate and the circuit portion.
In this arrangement, the method further includes mounting the die of the liquid crystal display device to a substrate, and affixing a distal coupling region of the tape member to the substrate. The electrically connecting may further include thermally bonding the contact portion to transparent plate.